This invention relates to weight measuring systems and more particularly to apparatus for indicating when each scale in a plural scale system such as a parts counter is zeroed.
Various types of systems involve the use of a plurality of scales for supplying weight data. One such system is a parts counter which determines a count of the number of parts in a container based upon the total net weight of the parts in the container and an average weight for such parts. A system of this type is typically provided with a minor or sample scale which measures the weight of a predetermined number of such parts and with a major scale which measures the weight of the parts to be counted. Computing apparatus then computes an average part weight from the total weight of the sample parts and the number of sample parts placed on the sample scale. Subsequently, the average part weight is divided into the net weight of the parts to be counted to obtain a count of the number of parts on the major scale. Parts counters of this type are generally provided with either a single display for selectively indicating the computed count, the weight of the parts to be counted and the weight of the sample parts, or the apparatus may be provided with individual displays for this data.
Other types of multiple scale systems also exist. A batch weighing system may, for example, include a plurality of weight measuring scales connected to a single batch process controller. The controller may include only a single indicator for displaying the weight on a selected one of the scales or a plurality of indicators for simultaneously indicating several weights.
One problem commonly encountered with plural scale systems is in correcting the weight output of each of the scales for zero error under a no load condition and in providing suitable means for indicating when one or more of the scales is not properly zeroed. There is in particular a problem in indicating the presence of zero error when a plurality of scales are selectively connected to only a single weight indicator. In systems of this type, an indication of whether or not a scale is properly zeroed has been provided only for the scale currently connected to the weight indicator. In many systems, such as in the piece counting system disclosed in U.S. Pat. No. 3,716,706 which issued on Feb. 13, 1973 to Alden J. Gray, two separate scales are selectively connected to a digital weight indicator on the piece counting apparatus. The individual scales are corrected for zero errors under a no load condition by means of a summing amplifier and a variable voltage source for each scale. Both the scale output and the variable voltage are connected to inputs to the summing amplifier. The scale is then connected through the summing amplifier to the digital display and the variable voltage source is adjusted until the display indicates zero for a no load condition. Each scale must be individually zeroed since only one scale is connected to the display at any given instant. While a scale is connected to the display, there is no indication whether or not a load is present on the other scale and, if a load is not present, whether or not the scale is zeroed. Even though a scale may have been zeroed at some time in the past, it does not necessarily remain zeroed due to changes in temperature, voltage fluctuations and changes in the operating parameters of various circuit components caused by, for example, component ageing. It is therefore desirable to have circuitry for automatically zeroing each of a plurality of scales and, at the same time, for indicating whether or not each scale is actually zeroed.